Stepping motors conforming to the general definition given above are used, for example, in electronic timepieces for driving their display members which usually are indicator hands.
U.S. Pat. No. 4,371,821 describes such a stepping motor which comprises, as an example, a stator having three pole faces defining therebetween a substantially cylindrical space receiving the permanent magnet of the rotor whose axis of rotation coincides substantially with the axis of this cylindrical space.
The permanent magnet of the rotor has an axis of magnetization intersecting the axis of rotation at an angle of about 90.degree. .
Each arcuate pole face extends over about 120.degree. in plan view of the motor and is situated at a first end of a pole piece.
Each of these first ends of the pole pieces is connected to the first end of the two other pole pieces by zones of high reluctance situated on either side of the corresponding pole piece.
The second ends of the first and the second pole pieces are connected by a first core on which is wound a first coil, and the second ends of the first and the third pole pieces are also connected by a second core on which is wound a second coil.
Finally, the stator of the motor comprises means for producing a positioning torque of the rotor which tends to maintain it in or return it to either of two rest positions. These rest positions are those in which the axis of magnetization of the permanent magnet has a direction designated as the rest direction which is the direction of a straight line cutting the axis of rotation of the rotor at an angle of about 90.degree. and passing through the middle of the first pole face. This straight line is thus the axis of symmetry of the first pole face, as well as the axis of symmetry of the second and third pole faces relative to one another.
The magnetic fields produced by the coils when a current flows therethrough thus assume, in the cylindrical space defined by the pole faces, directions symmetrical to one another relative to this axis of symmetry.
In another embodiment described in the above-mentioned U.S. Pat. No. 4,371,821, the stator of the motor does not have a pole piece and the two coils, without a core, have the shape of substantially flat frames partially surrounding the rotor's magnet.
The planes of these coils form an angle whose bisecting plane contains the axis of rotation of the rotor.
The directions of the magnetic fields produced by these coils are thus symmetrical in relation to an axis of symmetry in this bisecting plane and perpendicular to the axis of rotation of the rotor.
In this embodiment too, the motor includes means for producing a rotor-positioning torque tending to maintain the rotor in or to return it to either one of two angular rest positions which be are the angular positions in which the axis of magnetization of the rotor's permanent magnet has a rest direction which is the direction of the above mentioned axis of symmetry.
The aforementioned U.S. Pat. No. 4,371,821 also describes a method of controlling the above-described motor.
This method consists in delivering to the coils drive pulses made up of two parts. During the first part of each drive pulse, the coils are supplied with voltages which are equal in absolute value and have a polarity such that the magnetic field applied to the rotor's magnet, and which is the resultant produced by the addition of the magnetic fields produced by these coils, has a direction perpendicular to the above-defined rest direction and a sense such that the rotor begins to turn in the desired sense of rotation. At the end of the first part of each drive pulse, the polarity of the voltage supplied to one of the coils is reversed in such a manner that the resultant magnetic field has a direction parallel to the rest direction and a sense opposite to the sense that the axis of magnetization of the rotor's magnet had before the drive pulse began.
Each of these two parts of the drive pulse lasts for some milliseconds.
This motor has the advantage, all other factors being equal, of producing an identical torque in both senses of rotation.
However, for the motor to operate properly and at maximum efficiency, it would be necessary for the second part of the drive pulse to begin when the rotor has turned by exactly 90.degree. .
This is hardly ever achieved in practice, because the angle through which the rotor turns during the first part of the drive pulse depends on the applied resisting torque that has to be overcome.
When the motor's coils are supplied in the way that they are supplied during the first part of the drive pulse, the drive torque has its maximum value when the rotor occupies one of its rest positions and drops rapidly when the rotor begins to turn. Moreover, when the coils are supplied in the way that they are supplied during the second part of the drive pulse, the drive torque is zero when the rotor occupies one of its rest positions and increases quite slowly as a function of the angle of rotation of the rotor.
At the beginning of the second part of the drive pulse, the torque produced by the motor thus has a value which is only a fraction of the maximum torque it can produce, and this fraction decreases as the opposing or resisting torque applied to the rotor increases.
As a result, the efficiency of the motor is quite low and, to drive a given mechanical load, it consumes more electrical energy than a conventional stepping motor rotatable in only one direction.
Furthermore, if the opposing torque applied to the rotor is low, the rotor reaches the position where it has turned by 90.degree. before the first part of the drive pulse has finished.
The electrical energy delivered to the motor during the end of this first part of the drive pulse is dissipated as pure loss, which further decreases the efficiency of this motor.
Moreover, after having reached the above-mentioned position, the rotor oscillates about this position until the end of the first part of the drive pulse. There is therefore a non-negligeable risk that the second part of the drive pulse drives the rotor in a sense opposite the desired sense instead of making it finish its step correctly.
Reliable operation of the motor therefore cannot be guaranteed.
Additionally, because it is necessary to reverse the sense of the current in one of the coils during each drive pulse, it is difficult to combine the above-described control circuit of the motor with a circuit of well known type for reducing the consumption of a stepping motor by adjusting the duration of the supplied drive pulses to the mechanical load it drives.
U.S. Pat. No. 4,514,676 proposes a method of controlling the above-described motor that avoids some of these drawbacks.
According to this method, one of the coils is supplied, alone, for turning the rotor in one sense, and the other coil is supplied, alone, for turning the rotor in the other sense.
For each of the rest positions of the rotor, the polarity of the voltage delivered to the coil corresponding to the desired sense of rotation is selected in such a manner that the magnetic field applied to the rotor's magnet makes, when this rotor is in this rest position, an angle of about 120.degree. with the rest direction of the rotor.
When this motor is controlled according to this method, the torque it produces begins by increasing, goes through a maximum value when the rotor has turned through about 30.degree. and then decreases.
This variation of the drive torque as a function of the angle of rotation of the rotor represents an improvement over the case where the motor is controlled in the manner described in the above-mentioned U.S. Pat. No. 4,371,821. Also, most of the above-mentioned drawbacks are obviated.
Nevertheless, because of the fact that only a single coil is supplied, and all other factors being equal, the losses in this coil by the Joule effect are higher than those produced when the motor is controlled in the manner described in the above-mentioned U.S. Pat. No. 4,371,821. Therefore, the efficiency of the motor is not substantially improved by the latter control method.
U.S. Pat. No. 4 546 278 also describes a motor corresponding to the above definition. This motor and its control method will not be repeated here except to mention that this method has about the same drawbacks as the method described in U.S. Pat. No. 4,371,821.